1. Field of the Invention
The present invention relates generally to electronic circuits, and more particularly, to a method of compensating output voltage distortion of a half-bridge inverter and a device based on the method.
2. Description of the Related Art
Referring to FIG. 9, a circuitry of a conventional half-bridge inverter includes a direct current (DC) voltage source VDC, an upper capacitor C1 and a lower capacitor C2, both of which are serially interconnected, two active switches S1 and S2, and an output end. The output end has a filter circuit composed of an inductor LO and a capacitor CO. A general half-bridge inverter is controlled by sinusoidal pulse width modulation and the two switches S1 and S2 are complementarily switchable.
During the positive half cycle of the output voltage of the above-mentioned half-bridge inverter, there are two operational statuses as follows.
1. The Switch S1 Being Closed-Circuit and the Switch S2 Being Open-Circuit
As shown in FIG. 10, while the energy of the DC voltage source VDC is transmitted to the output end through the switch S1, an inductor current iLO rises linearly and the cross voltage at two ends of the inductor LO is the difference between the cross voltage of the upper capacitor and the output voltage during the positive half cycle. According to the inductor voltage-current relationship, the larger the cross voltage at the two ends of the inductor is, the larger the rising slope of the output inductor current of the inverter becomes.
2. Both of the Two Switches Being Open-Circuit
As shown in FIG. 11, because of continuity, the inductor current passes through a parasitic diode of the switch S2 to generate a discharge path and the current lowers gradually. If the switch S2 is turned on, the current path will not change. While the switch S2 is open-circuit and the switch S1 is closed-circuit again, the half-bridge inverter returns to the first operational status.
During the negative half cycle of the output voltage of the above-mentioned half-bridge inverter, there are also two operational statuses as follows.
1. The Switch S1 Being Open-Circuit and the Switch S2 Being Closed-Circuit
As shown in FIG. 12, except the different direction that the inductor current iLO flows, it is the same as the status indicated in the first operational status during the positive half cycle of the output voltage of the half-bridge inverter.
2. Both of the Two Switches Being Open-Circuit
As shown in FIG. 13, except the path that the inductor releases energy is changed to the discharge path that the inductor current passes through a parasitic diode of the switch S2, it is the same as the status indicated in the second operational status during the positive half cycle of the output voltage of the half-bridge inverter.
Because the waveform of the output voltage of the half-bridge inverter is derived from the duty ratio of a regulating switch, while the output voltage is positive, the duty ratio of the upper switch S1 becomes large; meanwhile, the discharge time of the upper capacitor C1 becomes long and the charge time of the same becomes short. Such repeated operations keep the capacitor voltage downgrading. Further, the input DC voltage is the sum of the voltages of the two capacitors, such that the voltage of the lower capacitor C2 rises to cause capacitor voltage unbalance between the upper and lower capacitors.
When the capacitor voltage unbalance reaches a considerable extent, one of the capacitors generates distortion resulted from undervoltage to further cause distorted output voltage. If the waveform of the output voltage is a sinusoidal waveform originally, however while affected by the output voltage distortion, the waveform of the output voltage will become a distorted sinusoidal one.
Conventionally, there are some methods of balancing the voltage of the capacitors of the half-bridge inverter for solution to the distorted waveform of the output voltage resulted from the capacitor voltage unbalance. The methods include:
1. Enlarging Capacitance                Enlarging the capacitance can relieve and even prevent the capacitor voltage unbalance and thus the waveform of the output voltage avoids distortion. However, it will increase the size and the cost of the capacitor.        
2. Increasing Output Frequency                Increasing output frequency can shorten the time of the positive/negative half cycle to further shorten the time that one of the upper and lower capacitors provides electric energy such that the capacitor voltage falling degree can be reduced to further relieve and even avoid the capacitor voltage unbalance, thus preventing the waveform of the output voltage from distortion. However, it is not applicable to the low output frequency. Further, the change of the output frequency specification is impractical.        
3. Adopting Passive Capacitor Voltage Balancing Circuit                Respective two ends of the upper and lower capacitors are directly connected in parallel with resistors having the same resistance. According to the voltage divider theorem, the respective voltages of the resistors connected in parallel are the same, such that the capacitor voltage will be the same as the voltage of the resistor to ensure the balance of the respective voltages of the upper and lower capacitors. However, the resistors connected in parallel will produce power consumption and reduce the efficiency. If the resistance of the parallel resistors is too large, the improvement will not be significant. If the resistance of the parallel resistors is small, the power consumption will be highly increased.        
In addition, U.S. Pat. No. 6,314,007 disclosed a compensating technique which starts with setting voltage level, switching an auxiliary switch while the capacitor voltage is higher or lower than the voltage level, and then the current flows through the auxiliary inductor to charge the upper and lower capacitors to ensure the capacitor voltage balance between the upper and lower capacitors. It can prevent the influence of the capacitor voltage to the output voltage waveform.
As disclosed in the aforementioned prior art, it was necessary to set the voltage level to balance the capacitor voltage to prevent the waveform of the output voltage from distortion. However, while the load or the output frequency is different, it is necessary to set different voltage level. If the range of the voltage level is too small, the auxiliary switch will begin switching before the waveform of the output voltage becomes distorted. It will cause unnecessary switching losses to reduce the overall efficiency of the inverter. On the contrary, if the range of the voltage level is too large, the auxiliary switch may be too slowly activated to still cause distorted waveform of the output voltage due to the capacitor voltage unbalance.
Therefore, when the output terminal of the half-bridge inverter is connected to variable load, e.g. the amplitude or the frequency is variable, the aforementioned prior art fails to take care of the quality of the waveform of the output voltage and the efficiency of the inverter at the same time.